1. Field of the Invention
This invention relates to an LED (Light Emitting Diode) based lamp and in particular, although not exclusively, to such a lamp that can be used as a direct replacement for a filament (incandescent) lamp in a recessed lighting fixture such as a ceiling mountable downlight.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip generates blue light and the phosphor material(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor material provides light which appears to the human eye as being nearly white in color.
Currently there is a lot of interest in using high brightness white LEDs to replace conventional incandescent light bulbs, halogen reflector lamps and fluorescent lamps. Most lighting devices utilizing high brightness white LEDs comprise arrangements in which a plurality of LEDs replaces the conventional light source component and utilize the existing optical components such as a reflector and/or a lens. Ideally a lamp for a downlight would generate an illuminance (luminous flux (power) per unit area incident on a surface) that was substantially uniform across the lamp's emission angle (beam spread). However, as light emission from a lamp is confined within a selected emission angle this can result in a greater proportion of the light emission being concentrated on the axis thereby further reducing illuminance uniformity within the emission angle. Unlike a filament lamp which closely approximates to a point source, LED based lamps generates light which is often far from point source in character requiring the development of new optical arrangements for LED lamps for general lighting applications. A need exists for an LED based lamp with a selected emission angle and whose illuminance is more uniform than the known lamps.